1. Field of the Invention
The present invention relates to a soft cold-rolled steel sheet and a method for making the same.
2. Description of the Related Arts
In conventional production of cold-rolled steel sheets for working which are produced by continuous annealing, high-temperature coiling has been performed in the hot rolling in order to prompt precipitation of AlN and coarsening of carbides and thus to achieve softening and high r-values. High-temperature coiling, however, causes an increased scale thickness at both ends of the coil by oxygen which is readily supplied, and thus causes deterioration of acid pickling characteristics. As a method for decreasing a coiling temperature using softening by boron addition, unexamined Japanese Patent Publication No. 2-263932 discloses a method for making a cold-rolled steel sheet for deep drawing, in which a boron containing steel having a specified Mn/S ratio is heated to 1,000.degree. C. to 1,200.degree. C., coiled at 560.degree. C. to 650.degree. C., and continuously annealed at a relatively high temperature of 730.degree. C. to 880.degree. C. Various methods using excellent grain growth characteristics of boron containing steels have been proposed for achieving excellent workability by high-temperature continuous annealing after low-temperature coiling. For example, unexamined Japanese Patent Publication No. 7-3332 discloses a method for making a cold-rolled steel sheet for working which is characterized in that a boron containing steel sheet is coiled at 600.degree. C. to 700.degree. C., and annealed at 740.degree. C. to 930.degree. C. Unexamined Japanese Patent Publication No. 9-3550 discloses a method for making a cold-rolled steel sheet for working which is characterized in that a boron containing steel sheet is coiled at 630.degree. C. to 720.degree. C. and annealed at 800.degree. C. to 880.degree. C. Also, unexamined Japanese Patent Publication No. 56-156720 discloses a method for making a cold-rolled steel sheet having excellent workability in which the relationship between B and N is specified and high-temperature annealing is performed after low-temperature coiling at 650.degree. C. or less. Among methods which specify the B/N ratio, added elements, and/or the heating temperature of the slab in order to achieve more excellent workability, unexamined Japanese Patent Publication No. 64-15327 discloses a method which specifies the heating temperature of the steel slab containing B in an amount of higher than the equivalent of N, that is, coiling at 550.degree. C. to 700.degree. C. and annealing at 750.degree. C. to 850.degree. C.; and unexamined Japanese Patent Publication No. 61-266556 discloses a cold-rolled steel sheet having excellent press workability in which a steel containing 0.10 to 0.30% of Cr and having a B/N ratio in a specified range from 0.5 to 2.0 is coiled at 550.degree. C. to 700.degree. C. and annealed at approximately 800.degree. C.
When a boron containing steel having excellent grain growth characteristics is annealed at a high temperature of 700.degree. C. or more, a mixed grain texture will often form and thus surface quality will deteriorate during the working. In recent years, high-quality surface characteristics have been increasingly required. Deterioration of surface characteristics due to the mixed grain texture, which was out of consideration, is raising problems; however, the above-mentioned conventional technology do not teach a countermeasure against the decreased surface quality due to the mixed grain texture formed by annealing at 700.degree. C. or more.
As described above, there has not been a method for enhancing stability of the texture in a B containing steel during continuous annealing in order to prevent the formation of a mixed grain texture.
Thin steel sheets used in automobiles and home electric products require high formability, and achievement of softening and a high r-value is in intensive progress. When such a thin steel sheet having high formability is made by continuous annealing using a low-carbon aluminum-killed steel, C and N must be fixed as coarse precipitates by high-temperature coiling in hot rolling. Since the ends of the coil in the longitudinal direction (the T section: the top section of the coil, and the B section: the tail section of the coil) and the ends in the width direction have high cooling rates by direct contact with air even in the high-temperature coiling, AlN does not sufficiently precipitate. Since the unprecipitated AlN finely precipitates in continuous annealing, the ends in the longitudinal and width directions are hardened compared with the central section of the coil, resulting in so-called coil end characteristics. The high-temperature coiling also causes decreased acid pickling characteristics due to an increased scale thickness. As a method for solving such coil end characteristics and acid pickling characteristics, unexamined Japanese Patent Publication No. 48-100314 discloses a method for reducing the coiling temperature by the addition of B which react with N to form coarse BN and thus suppress the formation of fine AlN.
As described in unexamined Japanese Patent Publication No. 48-100314, improvement in the coil end characteristics is uniformly achieved by the addition of B, but a problem that the material quality varies arises.
In the conventional technology, the steel is hardened with an increased O content in the steel, and the material quality may vary even at the same O content in some cases.
In conventional production of cold-rolled steel sheets for working which are produced by continuous annealing, high-temperature coiling has been performed in the hot rolling in order to prompt precipitation of AlN and coarsening of carbides and thus to achieve softening and high r-values. High-temperature coiling, however, causes an increased scale thickness at both ends of the coil by oxygen which is readily supplied, and thus causes deterioration of acid pickling characteristics. Unexamined Japanese Patent Publication No. 48-100314 discloses a method for lowering the coiling temperature by fixing N with B as BN; however, application of this method to hot direct rolling does not cause effects by the lowered coiling temperature. In the heating furnace, a part of coarse MnS that precipitates in the slab is not solved. In contrast, in hot direct rolling, the rolling is performed in the state that MnS is entirely dissolved, hence fine MnS, which precipitates during the rolling, suppresses crystal grain growth.
For the purpose of obtaining a soft material by hot direct rolling having substantially the same quality as that by the heating furnace, unexamined Japanese Patent Publication No. 7-242995 discloses a method for softening by controlling the S content to 0.004% or less so as to reduce the fine MnS content. Unexamined Japanese Patent Publication No. 9-3550 discloses a method for prompting coarsening of the precipitate, in which a continuously cast slab is subjected to rolling before cooling to the Ar.sub.3 point or less so as to suppress the transformation of MnS, as nuclei of the precipitate, affected by the transformation of Fe before the rolling.
When the S content is reduced to 0.004% or less by the method disclosed in unexamined Japanese Patent Publication No. 7-242995, desulfurization costs are significantly high and thus the use is limited to high class steel sheets.
In the method disclosed in unexamined Japanese Patent Publication No. 9-3550, softening is not sufficiently performed and high-temperature annealing at 800.degree. C. or more is inevitable.
As described above, a method enabling low-temperature coiling in the hot direct rolling is now not developed when a soft cold-rolled steel sheet is produced.